The present invention relates to a method for containing or isolating thermally degradable toxic or radioactive wastes and, in particular, to an encapsulated hazardous waste pellet which is suitable for use as an aggregate in concrete mixture.
The need to safely dispose of hazardous wastes, including toxic and radioactive waste salts, is well known. Current attempts to make a safe wasteform for disposal of these hazardous wastes involve either (1) mixing particulate wastes or waste slurries into a portland cement-based mortar, or (2) drying the wastes and blending the solids with a thermosetting polymer such as asphalt or polyethylene. These techniques, however, are unsatisfactory.
Portland, cement-based wasteforms often have problems with the chemical interaction of the waste and the cement. For example, some wastes contain compounds that inhibit the hardening of the portland cement or compromise the strength of the resulting hydrated cement mixture. Further, although the waste-cement mixtures may form coherent masses, there is the problem of rainwater or groundwater infiltrating the mass and leaching soluble salts into the surrounding soil or groundwater.
Thermosetting polymer wasteforms such as asphalt-salt composites, while generally effective in preventing moisture from contacting the waste, cannot be used with waste salts that decompose when heated. Moreover, polymer-waste composites generally have to be enclosed in a container such as a steel drum for transportation and disposal. The drum adds to the cost of the wasteform and provides only temporary (15 to 20 year) containment if the drums are buried. Waste salts that are oxidizers (e.g. nitrates, chlorates) can form potentially flammable mixtures that can burn without access to air. Salts such as nitrates mixed with asphalt or plastic are the equivalent of a solid rocket fuel. If a drum of mixed organic compounds and oxidizers were to ignite, it would be very difficult to contain the resulting fire, and hazardous wastes would be released in the smoke and fumes produced.
When radioactive wastes are incorporated in asphalt or other organic polymers, the radiation breaks down the organic polymer and can generate hydrogen gas. If the wasteform is sealed in a drum, hydrogen gas can accumulate and pressurize the drum, creating a serious safety problem.
Organic polymer wasteforms are developed for long-term containment of waste which will be hazardous for hundreds of years. The possibility exists, therefore, that the wasteforms may be excavated at some future date when the nature of the material has been forgotten. If the wasteform is ever put to use where it can expose humans to hazards due to the waste's toxicity or radioactivity, a great deal of harm can be done unintentionally. Drums of hardened organic polymer may be inviting targets for reclamation in the future because they resemble uncontaminated material.
Concrete-based wasteforms have been favored over polymer-based wasteforms because the relatively high pH maintained in the concrete reduces solubility of many metal salts that are common in wastes and are a problem in the environment. For example, cadmium and lead are typically more soluble in-acid than in alkaline aqueous systems. It has not been possible to develop this type of chemical barrier with organic polymer encapsulation.
Typically, waste-asphalt composites (the most commonly used organic polymer system) will contain between 40% and 60% waste by weight In some cases it is useful to have an outer layer of uncontaminated material surrounding the waste to improve waste isolation and, in the case of radioactive wastes, to provide radiation shielding.
The present invention incorporates waste pellets coated with epoxy and powdered silicate in a concrete mixture. The powder coating of the pellets allows the pellets to be handled, separated, and distributed in the cementitious matrix. Without the coating, the pellets adhere to one another and cannot be efficiently distributed into the mixture. The coating of powdered silicate bonds to the surrounding concrete and creates an additional barrier to the movement of wastes out of the pellets.